Add Diagnostic class to represent a single diagnostic message

[jacob-carlborg]

The message is constructed on demand to be able to have the constructor pure and formatting the message in an impure way.

Extracted from #9480.

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[thewilsonator]

dsymbol/src/dsymbol/symbol.d(204): Deprecation: Cannot use `alias this` to partially initialize variable `part` of type `SymbolOwnership`. Use `part.ptr`

[wilzbach]

No, the error is:

dmd/errors.d(77:24)[warn]: Useless final attribute, private functions declared within a class are never virtual

[jacob-carlborg]

Sounds more reasonable. Will fix

[jacob-carlborg]

Fixed.

[WalterBright]

I want to put a hold on all this refactoring. I do not agree with the premise of it, and would like to revert #9350

I do not understand the value in adding hundreds and hundreds of lines of code that do next to nothing. What problem is it solving?

[jacob-carlborg]

#9350 (comment)

[jacob-carlborg]

#9480 (comment)

[wilzbach]

Rebased to upstream/master fix CircleCi.

[WalterBright]

Not doing this, for reasons explained elsewhere.

[jacob-carlborg]

By collecting all the diagnostics, instead of reporting them directly, and pushing up to the caller it will be possible to make the lexer pure. That means it no longer relies global state meaning it increases encapsulation.

[adamdruppe]

One thing I would really like long term is for all errors to be jam-packed with useful semantic information, such that they can be formatted in different contexts to best highlight that. (This is what I really mean when I say "xml error messages" on the forum, etc.)

I actually think something like this is the logical first start: error messages represented not as strings, but as objects. It just wraps a string for now, but ultimately if this leads to those input strings all being eliminated (it can still output those strings though, but the input should put the details in more structure), I'd see it as being a good thing.